Unexpected Effects of Fungicides on Cereal Yields1

Field experiments with barley have generally shown a good relationship between disease and yield. However, in some experiments the yield response is greater than would be expected from the level of disease. In contrast, other trials have shown a reduction of yield following applications of tridemorph to spring barley. On winter wheat, trials in Germany have shown a poor relationship between disease incidence and yield following application of carbendazim or captafol at GS 30–31. About 10 96 of these responses were negative, although there was an average yield increase of 2–3 %. Similarly, in a series of trials in Britain an average yield increase of 3 % was not related to disease level. In these trials about 25 % of results showed no increase in yield. The carbendazim fungicides, and also certain others, have some cytokinin-like activity and it is possible that this is related to extended green leaf survival. Other interactions with host-cell chemistry can be envisaged. Fungicides also influence stability of the leaf microflora. By this effect they may favour diseases such as Typhula and Rhizoclonia, or else suppress secondary pathogens (Alternaria, Cladosporium) at the end of the season.